1. Field of the Invention
The invention relates to a method of anisotropic plasma etching of substrates preferably defined with an etching mask in which the etch rate and selectivity is increased. The method can be well implemented for manufacturing microelectromechanical system (MEMS), as well as microelectronic devices.
2. Background of the Related Art
Anisotropic plasma etching, particularly for single crystal silicon, can work independent of crystal orientation of the substrate or doping level. This method also applies to doped or undoped polysilicon. Preferred fields of applications are MEMS technology, where structures have a high aspect ratio, i.e., a high structural height to width ratio. Other examples include surface wave technology, where narrow grooves and vertical walls are etched to produce actuators, surface wave filters, delay lines, etc. Additional microelectronics applications include storage cells, insulation, collector contacts, etc.
The Reactive Ion Etching (RIE) processes which are commonly used for anisotropic silicon etch employ relatively high energy ions (xe2x89xa7100 eV) and reactive halogens, such as fluorine, chlorine or bromine, which are used directly in the plasma or are released from corresponding compounds, like CF4, CF3Br, C2F6, CCI4, CHCI3. The resulting ion bombardment of the etching ground, i.e., the area to be etched, initiates the reaction of the radicals with the silicon to be depleted. The etching of the sidewalls is minimal due to the directionality of the ions.
Problems occur when, to increase the speed of silicon removal (i.e., the etch rate), one tries to enhance the plasma density by increasing the power coupled to the plasma discharge. This can be accomplished by either increasing the power of the source for the plasma discharge or by increasing the value of the polarization voltage applied to the substrate. However, as the power is increased more hot ions are produced and the direction of ion movement becomes more random. The results is that more ions and radicals are depleted by the walls of the trenches, with the inevitable loss of anisotropy of the etch. To overcome this problem, one must reduce the etch rate, resulting in the loss of the throughput.
An additional problem encountered is mask degradation. As etch rate is reduced, etch time, and therefore mask exposure time, are increased, leading to more rapid mask degradation, i.e., reduced selectivity.
U.S. Pat. No. 5,501,893 discloses an etching method that includes alternating etching and polymerizing steps where the purpose of the polymerizing step is to provide a polymer layer on the surfaces that were exposed in the previous etching step to form a temporary etch stop. Thus the side walls are protected from etching during the etching steps. However, the gas mixtures introduced during the etching and polymerizing steps are different such that different gas mixtures are cycled during the respective etching and polymerizing steps. In the etching step the gas mixture includes SF2 and Ar and in the polymerizing step the gas mixture includes CHF3 and Ar.
The problem with cycling different gases is that the time ratio of the etch deposition cycle depends on the speed of the gas mixtures and varies from point to point, affecting the uniformity. Also, the time for species to arrive at the bottom of the etched trench varies drastically for trenches having different sized openings. Also, this method typically requires more complex hardware and controls to introduce the two different gas mixtures in cycles.
It is desirable to make the plasma as cold as possible with coexisting polymer-producing unsaturated monomers and fluorine, bromine or iodine radicals. The energy level sufficient for ionization is different for each gas. In certain cases, the activation energy for polymer-producing gases (C4F8, CHF3) is two times higher than for radical producing gases (SF6). The object of the invention is to establish a method for enhancing the treatment of the silicon surface being etched, by using the differences in the energies of activation of the reactive gases to arrive at optimal conditions for both etching and passivation, and alternating those conditions at a high rate to produce a high aspect ratio, and high selectivity etch process.
The object of the present invention is accomplished by providing a method of anisotropic plasma etching of substrates (typically silicon) comprising the following steps:
a) placing the substrate with the surface to be selectively etched on an electrode connected to an electromagnet power source;
b) introducing mixed gases consisting of an etching gas (SF6) and a passivation gas (CHF3, C4F8, etc.) into the processing chamber;
c) exciting the mixed gases with lower power (100-800 W) electromagnetic radiation sufficient to produce a plasma containing ions and radicals for etching;
d) concurrent with step (c), applying high polarizing voltage (50-500 eV) to the substrate via its electromagnet power source to produce a highly anisotropic etch;
e) exciting the mixed gases with high power (1000-3000 W) electromagnetic radiation to produce in the plasma unsaturated monomers for protective polymer coating formation;
f) concurrent with step (e), applying low polarizing voltage (0-25 eV) to the substrate to form a conformal polymer coating on the exposed side walls of the surfaces being etched; and
alternating steps c) and d) with steps e) and f) to achieve an anisotropic etch with a high etch rate and selectivity than is currently being achieved using other methodologies.
The method used in this invention enables the substrate to be etched without using helium gas as a cooling medium. This is because lower power is used to excite the etching gas resulting in less heat being generated during the process.
A further advantage of this invention is that a constant flow of mixed gas is injected into the process chamber during processing, resulting in a process that is more stable and repeatable.